1. Field of the Invention
The present invention relates to a small projection lens that enlarges and projects, for example, display information from a light valve, such as a transmissive or reflective liquid crystal display device or a DMD (digital micro-mirror device), and more particularly, to a small projection lens applicable for a so-called handy-type projection display device having high portability, and a projection display device using the same.
2. Description of the Related Art
Projection display devices using light valves, such as a liquid crystal display device or a DMD display device, have come into widespread use. As the size of the light valve is reduced, the precision thereof is improved, and personal computers are widely spread, the use of the projection display device for presentation is increased. Therefore, in recent years, there is a demand for, particularly, a small projection display device with high portability. In addition, it will be considered that a projection display device is treated with the same sense as treating a mobile phone or a flashlight in the near feature. Therefore, it is necessary to further improve portability.
In order to meet the requirements, particularly, it is effective to reduce the thickness of an apparatus in a direction vertical to the optical axis of a projection optical system, generally, in the thickness direction of a case of the apparatus. In order to reduce the size of the apparatus, it is important to reduce the outside diameters of all lenses in a projection lens.
For example, JP-B-3508011 and JP-A-2005-84456 disclose projection lenses that have a small size and are capable of improving the portability of an apparatus. In the projection lens, a reduction side is telecentric, and a certain amount of space to perform color composition or separate illumination light from projection light is ensured on the rear side of the lens.
However, the projection lens disclosed in JP-B-3508011 and JP-A-2005-84456 has a small angle of view of about 50 degrees, and it is difficult to project an image such that an image size is increased at a short distance.
As described above, in the projection lens disclosed in JP-B-3508011 and JP-A-2005-84456, the reduction side is telecentric and a certain amount of space is ensured on the rear side of the lens. However, JP-B-3508011 and JP-A-2005-84456 do not disclose any measures to prevent an increase in the size of a reduction-side lens and to actively reduce the size of the projection lens. In addition, JP-B-3508011 and JP-A-2005-84456 do not disclose any measures to actively reduce the sizes of the reduction-side lens and a magnification-side lens with respect to an effective light beam.
In order to improve the portability, it is effective to reduce the size of a light valve. However, JP-B-3508011 and JP-A-2005-84456 do not disclose this structure.